Two chitin synthetases, Chsl and Chs2, are known to be present in yeast. Chs2 is essential for septum formation and cell division. The previous hypothesis, that Chsl has a repair function during cell separation, has been now confirmed by unambiguous genetic evidence. The genes coding for the two chitin synthetases, CHSI and CHS2, share an extensive homologous region but differ in the amino terminal sequence. By gene deletions and ligations we have determined that the amino terminal region of CHS2 is not required for either enzymatic activity or function. Genetic analysis has confirmed that proteinase B is not required for activation of the Chsl zymogen in vivo. Chsl has been found to be more sensitive than Chs2 to certain antifungal agents.